A compound waveguide device contains a first optical waveguide, and a second optical waveguide placed in optical proximity to at least a portion of the first waveguide. The second waveguide (possibly electro-optically tunable), placed in optical proximity to the first waveguide, can be used for the coupling of optical energy, within a selectable spectral band, from the first waveguide to the second. The coupling effect between the first and second waveguides can be utilized in a variety of optical signal processing applications.
For example, in some filtering applications, efficient recollection of the coupled optical energy for further use (e.g., in signal detection or demodulation, source stabilization feedback) is required. Multiple wavelength bands may be transmitted through the first waveguide, in a wavelength division multiplexing (WDM) system, in which case the second waveguide can be used as a filter to extract information carried in one of the bands. If an electro-optic material (e.g., LiNbO.sub.3) is used to form the second waveguide, then the device can be configured as an active, electro-optically tunable filter. In another embodiment, the geometric and physical properties of the second waveguide itself may result in a useful passive filter inherently tuned to a particular wavelength of interest.
In another example, if an electro-optic material is used to form the second waveguide, the device can be configured as an intensity modulator for a fixed wavelength signal transmitted through the first waveguide. By applying an electric field to the second waveguide using, for example, a suitable high-speed electrode pattern, a refractive index change is induced in the second waveguide and a corresponding shift in the spectral response results. The transmitted intensity of the signal in the first waveguide can therefore be modulated by the shifting spectral response, resulting in a modulator which can operate up to microwave frequencies.
Depending upon the particular application, different coupling characteristics between the waveguides are required. However, certain improvements to these characteristics are commonly desired in many applications. Diffractive losses in the second waveguide itself, while in certain applications useful, in others may adversely affect the shape of the spectral response of the device. For example, eliminating diffractive losses is often desirable because recollection efficiency in a signal detection or demodulation device can be thereby improved.
The operation of the device is determined by its intrinsic spectral response and the electric field applied to the second, electro-optically tunable waveguide. In a filtering context, sharpening the coupling resonance, in either a bandpass or bandstop configuration, improves the spectral selectivity of the device. In a modulation context, steepening the edge of the coupling resonance can increase a modulation of an optical carrier transmission for a given amount of device tuning about the carrier wavelength. Therefore, for a given transmission modulation, steepening the coupling resonance lowers the operating voltage of the device. In either context, altering the geometry of the second, electro-optically tunable waveguide to increase the applied electric field and its distribution therein increases device tunability, and thereby improves its performance.
Therefore, what is required, is a compound waveguide device with improved tunability and which offers other improvements to known devices. In the filter context, preventing unnecessary diffractive losses in the second waveguide improves spectral selectivity and secondary coupling. In the modulator context, steepening the resonance edge increases device responsivity which allows for lower drive voltages.